This invention relates in general, to a secure telephone system and more particularly to a security system to prevent unauthorized use of a cordless telephone system.
Recently the F.C.C. designated five frequencies to be used for the transmission of signals in a cordless telephone system. These frequencies are in the previously unallocated band at 49 MHz. The frequencies employed in this band are 49.83, 49.845, 49.860, 49.875 and 49.890 MHz. The purpose of the allocation is to implement cordless telephone operation. Essentially such systems are in widespread use and an example of such a system is manufactured and sold by I.T.T., the Consumer Speciality Products Division (CPSD) of Clark, N.J., under the Model No. PC1800. Cordless telephone systems are also made and supplied by a host of other companies.
The systems basically operate as follows:
A cordless telephone system uses a base station which is capable of full duplex operation with a portable hand held remote unit. The base unit is coupled to a user's telephone. When the telephone rings the base station detects ringing and transmits a ring signal to the remote unit using one of the above frequencies. The user having the remote unit receives the signal and returns an "answer" signal to the base unit, which now causes the telephone subset to go "off-hook." The user can now conduct a full conversation from the remote unit to the base unit and therefore through the telephone based on the transmission of signals between the base unit and the remote unit. Such systems also allow the user to initiate a call from the remote unit. To do this a service request is sent to the base unit which in turn transmits dial tone if the telephone is not busy. The remote unit contains a rotary or quick step dialer which enables the user to send dial pulses to the base unit which places the dial pulses on the telephone line to make a call in the same manner as is conventionally done. Hence the base section monitors the telephone line, detects dial tone and transmits the dial tone to the remote unit when a remote call is to be placed. These functions are carried out between the base unit and the remote unit at the allocated frequencies.
The typical distance for reliable transmission between a base station and a remote unit is about 1000 feet more or less depending on transmission conditions and other factors. Thus the cordless telephone system allows a user to be away from his phone, or by the pool or in the backyard with complete access to the phone according to operation. This provides great convenience for the user and the cordless telephone system is a highly successful product.
However, the advent of the cordless telephone system has created a substantial problem. Because the allocated frequencies only total five frequencies, may users are, in fact, assigned the same frequencies for transmitting and receiving. People who are cognizant of this fact have used the remote unit to place unauthorized telephone calls from the telephone lines of other users. For example, a person having a remote unit can access any base station operating on a compatible frequency. The person can therefore receive the dial tone and place a call on another person's telephone. If the person is intent in doing so he may further use an automobile and travel about with the remote unit to access other telephones operating on the same frequency. This problem is presently a serious one, but it will become even greater as more cordless systems are employed, as the likelihood of accessing the correct frequency greatly increases with the supply of additional units.
It is therefore an object of the present invention to provide apparatus operating in conjunction with the base station to mitigate against improper access as above described.
The apparatus provided is relatively inexpensive and reliable and is adapted to operate with most existing cordless systems as well as capable of being provided as an add-on to such systems.